1. Field of the Invention
The present invention relates to radio communication systems, and more particularly, to an RF transceiver that adaptively maintains power output level linearity across a broad spectrum of transmitting frequencies.
2. Description of Related Art
Spread spectrum modulation techniques are increasingly desirable for communications, navigation, radar and other applications. In a spread spectrum system, the transmitted signal is spread over a frequency band that is wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming, and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced; thereby reducing interference to other radio devices. In view of these significant advantages, spread spectrum communication systems are highly desirable for commercial data transmission.
In one type of spread spectrum communication system, a radio frequency (RF) carrier is shifted in discrete increments in a pattern dictated by a predetermined sequence. These spread spectrum systems are known as "frequency hopping" modulation systems, since the transmitter jumps from frequency to frequency in accordance with the predetermined sequence. Another type of spread spectrum communication system utilizes an RF carrier modulated by a digital code sequence having a bit rate, or chipping rate, much higher than the clock rate of the information signal. These spread spectrum systems are known as "direct sequence" modulation systems. The RF carrier may be modulated such that a data stream has one phase when a code sequence represents a data "one" and 180.degree. phase shift when the code sequence represents a data "zero." In either type of spread spectrum system, a power amplifier within the transmitter amplifies the spread spectrum-modulated signal to the required output level and applies the amplified signal to an antenna.
It is also known to operate a plurality of spread spectrum radio transmitters/receivers (hereinafter "transceivers") together in a wireless local area network (LAN). A central host processing unit could send information to and receive information from any one of the plurality of remotely disposed transceivers. In such a wireless LAN, the remote transceivers may comprise portable units that operate within a defined environment to report information back to the central host processing unit. It should be apparent that such wireless LAN systems offer increased flexibility over more traditional hard-wired systems by enabling operators of the remote transceivers substantial freedom of movement through the environment.
Ideally, the output level linearity of a power amplifier for a spread spectrum communication system should be maintained as flat as possible so that the transmitted power level at each frequency channel is equal to that transmitted for every other frequency. In a frequency hopping system, variations in signal power reduce the ability of the transceiver to interpret the information contained within a transmitted message as well as reducing the immunity of the system to interference or jamming. Moreover, the reduction in power output of some channels has the effect of reducing the usable range of a transceiver in a LAN environment. While this effect is applicable to all constant envelope or angle modulated RF transmissions, it is especially detrimental to frequency hopping systems in which the channels are changed constantly. Each time the transmitter changes to a frequency having a lower power output, the range and/or throughput of the entire LAN is reduced. Direct sequence systems may also operate in a number of separate channels, and variations in output levels between the channels can similarly reduce the range and throughput of such systems.
The procedures for obtaining regulatory agency approval tend to further exacerbate the amplifier output level linearity problem. Certain governmental agencies, such as the Federal Communications Commission (FCC), have regulations that govern the total allowable power output of a radio transmitter. In order to obtain agency approval, the output power of the channel with the highest amplitude is typically adjusted to operate at the maximum power level permitted under the regulations. Since the output power varies over the operating band, the other channels will necessarily fall short of the maximum level.
Two techniques are known in the art to compensate for the non-linearity of the power amplifier: network compensation and analog feedback. Network compensation is an attempt to attenuate the input frequencies at the receiver through the use of matching networks in order to flatten the power output levels across the channels. This technique is undesirable since it wastes the output power of the higher power level channels, while also increasing the complexity and part count of the receiver. In the analog feedback technique, the transmitted channel outputs are sampled in real time to derive a corresponding DC voltage. The DC voltage is then shifted to a desirable level and fed back directly into the power amplifier bias circuitry. This technique is undesirable since it fails to account for the unique operating characteristics of the transmitter, and also adds complexity to the transmitter due in particular to the complex transfer function generated in the feedback loop.
Thus, it would be desirable to provide a transmitter that provides a linear output power level across a broad bandwidth of frequencies without impacting either performance or efficiency.